Plasma lamp with dielectric waveguide integrated with transparent bulb

ABSTRACT

A dielectric waveguide integrated plasma lamp (DWIPL) with a body comprising at least one dielectric material having a dielectric constant greater than approximately 2, and having a shape and dimensions such that the body resonates in at least one resonant mode when microwave energy of an appropriate frequency is coupled into the body. A dielectric bulb within a lamp chamber in the body contains a fill which when receiving energy from the resonating body forms a light-emitting plasma. The bulb is transparent to visible light and infrared radiation emitted by the plasma. Radiative energy lost from the plasma is recycled by reflecting the radiation from thin-film, multi-layer coatings on bulb exterior surfaces and/or lamp chamber surfaces back into the bulb. The lamp further includes two- or three-microwave probe configurations minimizing power reflected from the body back to the microwave source when the source operates: (a) at a frequency such that the body resonates in a single mode; or (b) at one frequency such that the body resonates in a relatively higher mode before a plasma is formed, and at another frequency such that the body resonates in a relatively lower order mode after the plasma reaches steady state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.10/771,788 (“'788”) filed on Feb. 4, 2004, entitled “Plasma Lamp WithDielectric Waveguide,” which is a continuation of U.S. application Ser.No. 09/809,718 (“'718”) filed on Mar. 15, 2001 and issued as U.S. Pat.No. 6,737,809 B2, also entitled “Plasma Lamp With Dielectric Waveguide,”which claims priority to U.S. provisional application Ser. No.60/222,028 (“'028”) filed on Jul. 31, 2000, entitled “Plasma Lamp.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the present invention relates to devices and methods forgenerating light, and more particularly to electrodeless plasma lampsenergized by microwave radiation and having a solid dielectric waveguideintegrated with at least one transparent bulb, wherein heat energy fromthe plasma is recycled into the bulb(s), resulting in high efficiencyoperation.

2. Related Art

Our '718 application discloses a “dielectric waveguide integratedplasma” lamp (DWIPL) including a “dielectric waveguide,” viz., awaveguide coupled by a microwave probe to a source of microwave powerand having a body consisting essentially of dielectric material and aside with a lamp chamber extending into the body. The source operatingfrequency and waveguide body dimensions are selected such that the bodyresonates in at least one resonant mode having at least one electricfield maximum. The lamp further includes a bulb disposed within thechamber. Thus the body, chamber and bulb are integrated as a unitarystructure. The bulb contains a fill mixture (“fill”) that forms alight-emitting plasma when microwave power is directed by the waveguideinto the bulb. The '718 application also discloses a DWIPL including adielectric waveguide and two microwave probes. One probe, connected to afeedback means coupled between the probe and microwave source, probesthe waveguide body to instantaneously sample the field amplitude andphase and provides this information via the feedback means to the sourcewhich dynamically adjusts the operating frequency to maintain at leastone resonant mode within the waveguide body, thereby operating the lampin a “dielectric resonant oscillator” mode. The '718 application furtherdiscloses DWIPL embodiments which differ according to waveguide bodyshape, bulb type (a hermetically sealed envelope vis-a-vis a bulb whichis self-enclosed), number of bulbs (one vis-a-vis two), number of lampchambers (one vis-a-vis two), and number of probes (one vis-a-vis two).

A continuation-in-part application Ser. No. 10/356,340 (“'340”),published as Pub. No. 2003/0178943 A1 and entitled “Microwave EnergizedPlasma Lamp With Solid Dielectric Waveguide,” discloses advances indesign of the “drive probe” which supplies microwave power to the fill,and of the “feedback probe”, as well as utilization of a “start probe”to mitigate over-coupling of the drive probe, and amplifier and controlcircuits for two- and three-probe configurations which minimize powerreflected from the body back to the source both before a plasma isformed and after it reaches steady state. The '340 application furtherdiscloses techniques for sealing a waveguide body cavity (viz., a lampchamber) with a window or lens allowing seals to withstand largethermomechanical stresses and chamber pressures which develop duringlamp operation, alternative techniques for DWIPL assembly, and waveguidebodies having two solid dielectric materials.

The '718, '340 and '788 applications asserted that quartz bulbs areunsuitable for plasma lamps of the present invention because they wouldbe prone to failure in the 1000° C. temperature regime a bulb wallcontaining a plasma would experience and, even if structural failure didnot occur, would be unstable in their mechanical, optical and electricalproperties over long periods when repeatedly cycled in temperature. Theconclusion was that use of a quartz bulb would likely result in a lampprone to early failure. However, we have recently demonstrated thatquartz can be a suitable bulb material when used in the lamp embodimentsdisclosed herein, and moreover provides significant advantages that anopaque fill envelope or self-enclosed bulb cannot.

SUMMARY OF THE INVENTION

In one aspect a lamp according to the invention includes a waveguidehaving a body including at least one dielectric material with adielectric constant greater than approximately 2, and at least one bodysurface determined by a waveguide outer surface. The lamp furtherincludes a probe within the body coupling microwave energy into the bodyfrom a source operating in a frequency range from about 0.25 to about 30GHz. The body resonates in at least one mode having at least oneelectric field maximum. The body has a lamp chamber depending from thewaveguide outer surface, thus determining an aperture, and the chamberis determined by a bottom surface and at least one surrounding wallsurface. The lamp further includes a transparent, dielectric bulb withinthe chamber, and a fill within the bulb which when receiving microwaveenergy from the resonating body forms a light-emitting plasma.

In a second aspect a lamp includes a self-enclosed bulb closely receivedwithin a lamp chamber in a dielectric waveguide body. The chamber isdetermined by an aperture, and an enclosure determined by a bottomsurface and at least one surrounding wall surface. The bulb has acylindrical wall consisting of transparent dielectric material, attachedto a bottom consisting of the same material. The bulb wall has acircumferential upper edge hermetically sealed to a transparent window.The exterior surface of the bulb wall is in thermal contact with thechamber wall surface, and the exterior surface of the bulb bottom is inthermal contact with the bottom surface of the chamber.

In a third aspect a lamp includes a self-enclosed bulb disposed within alamp chamber in a dielectric waveguide body. The chamber is determinedby an aperture, and an enclosure determined by a bottom surface and atleast one surrounding wall surface. The bulb includes a cylindricalwall, consisting of transparent dielectric material, which extendsupwardly in a circumferential lip having opposed lower and uppersurfaces, and is attached to a transparent bottom. The lip lower surfaceis hermetically sealed to a bulb support structure circumscribing theaperture and attached to the waveguide body. The bulb further includes awindow hermetically sealed to the lip upper surface.

In a fourth aspect a lamp includes a lamp chamber in a dielectricwaveguide body, determined by an aperture, and a shaped surface boundinga surrounding wall of dielectric material and tapering symmetrically toa bottom. The lamp further includes a self-enclosed bulb having acylindrical, transparent wall attached to a bottom, and a windowhermetically sealed to the wall. The bulb bottom is attached by a firstadhesive layer to a ceramic pedestal attached by a second adhesive layerproximate to the chamber bottom. The chamber surface is shaped to directlight emitted by plasma in the bulb so as to satisfy ray-divergencespecifications levied by an optical system receiving the lamp's outputradiation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a sectional view of a dielectric waveguide integratedplasma lamp (DWIPL) including a waveguide having a body consistingessentially of solid dielectric material, integrated with a bulbenvelope having a transparent wall and containing a light-emittingplasma.

FIG. 2 illustrates a sectional view of a DWIPL having a self-enclosedceramic bulb separated from the waveguide body by a vacuum gap, and asapphire window.

FIG. 3A illustrates a sectional view of a DWIPL self-enclosed bulbhaving a cylindrical, transparent wall and transparent bottom in thermalcontact with a lamp chamber wall, and a window made of the same materialas the wall and bottom.

FIG. 3B shows the FIG. 3A bulb with a multi-layer dielectric coating onthe exterior surfaces of the bulb wall and bottom.

FIG. 4A illustrates a sectional view of a DWIPL self-enclosed bulbhaving a cylindrical, transparent wall extending upwardly in a lip and atransparent bottom, and separated from the waveguide body by an air gap,and a window made of the same material as the wall, lip and bottom.

FIG. 4B shows the FIG. 4A bulb with a multi-layer dielectric coating onthe exterior surfaces of the cylindrical bulb wall and bottom.

FIG. 4C shows the FIG. 4A configuration with a multi-layer dielectriccoating on the wall and bottom of the cylindrical lamp chamber.

FIG. 4D shows the FIG. 4A configuration with a multi-layer dielectriccoating on the exterior surfaces of the bulb wall and bottom, and on thelamp chamber wall and bottom.

FIG. 5A illustrates a sectional view of a lamp chamber, open to air,wherein a self-enclosed cylindrical, transparent bulb is mounted on apedestal attached to the lamp chamber bottom.

FIG. 5B is a detail view of the FIG. 5A bulb and pedestal.

FIG. 5C shows the FIG. 5A configuration with a multi-layer dielectriccoating on the lamp chamber surface.

FIG. 5D shows the FIG. 5A configuration with a multi-layer dielectriccoating on the exterior surfaces of the bulb wall and window.

FIG. 5E shows the FIG. 5A configuration with a multi-layer dielectriccoating on the exterior surfaces of the bulb wall and window, and on thelamp chamber surface.

FIG. 5F shows the FIG. 5A configuration with a partial-lens, including alens portion and a planar portion, covering the chamber aperture.

FIG. 6A is an elevational view of a bulb having a transparent,cylindrical wall concentric with and surrounded by an opaque dielectricsleeve split into two halves, and separated from the sleeve by an airgap. A multi-layer dielectric coating is on the interior surface of eachsleeve-half.

FIG. 6B is an exploded plan view of the FIG. 6A configuration.

FIG. 7A is an elevational view of a bulb having a transparent,cylindrical wall concentric with and surrounded by a transparentdielectric sleeve split into two halves, and separated from the sleeveby an air gap. A multi-layer dielectric coating is on the exteriorsurface of each sleeve half.

FIG. 7B is an exploded plan view of the FIG. 7A configuration.

FIG. 8A is an elevational view of a bulb having a transparent,cylindrical wall concentric with and surrounded by a transparentdielectric one-piece sleeve, and separated from the sleeve by an airgap. A multi-layer dielectric coating is on the exterior surface of thesleeve.

FIG. 8B is an exploded plan view of the FIG. 8A configuration.

FIG. 9 shows the reflectance spectra of a preferred multi-layer coatingconsisting of SiO₂, which is transparent for wavelengths in the range0.12-4.5 μm, for light incident normally and 30 degrees off normal.

FIG. 10A schematically depicts a DWIPL having a cylindrical body whereina bulb and a drive probe are located at the electric field maximum of aresonant mode.

FIG. 10B schematically depicts the FIG. 10A DWIPL wherein the bulb islocated at the electric field maximum of the FIG. 10A resonant mode, anda drive probe is offset from the maximum. The FIG. 10B probe is longerthan the FIG. 10A probe to compensate for coupling loss due to theoffset.

FIG. 11A schematically depicts a DWIPL having a rectangular prism-shapedbody wherein are disposed a bulb, and a drive probe and a feedback probeconnected by a combined amplifier and control circuit.

FIG. 11B schematically depicts a DWIPL having a cylindrical body whereinare disposed a bulb, and a drive probe and a feedback probe connected bya combined amplifier and control circuit.

FIG. 12 schematically depicts a first embodiment of a DWIPL utilizing astart probe. The DWIPL has a cylindrical body wherein are disposed abulb, a drive probe, a feedback probe, and the start probe. The feedbackprobe is connected to the drive probe by a combined amplifier andcontrol circuit, and a splitter, and is connected to the start probe bythe amplifier and control circuit, the splitter, and a phase shifter.

FIG. 13 schematically depicts a second embodiment of a DWIPL utilizing astart probe. The DWIPL has a cylindrical body wherein are disposed abulb, a drive probe, a feedback probe, and the start probe. The feedbackprobe is connected to the drive probe and the start probe by a combinedamplifier and control circuit, and a circulator.

FIG. 14A schematically depicts a third embodiment of a DWIPL utilizing astart probe. The DWIPL has a cylindrical body wherein are disposed abulb, a drive probe, a feedback probe, and the start probe. The feedbackprobe is connected to the drive probe and the start probe by a combinedamplifier and control circuit, and a diplexer.

FIG. 14B schematically depicts an alternative configuration of the FIG.14A embodiment wherein the feedback probe is connected to the driveprobe by a diplexer and a first combined amplifier and control circuit,and to the start probe by the diplexer and a second combined amplifierand control circuit.

FIG. 15A schematically depicts a DWIPL wherein a start resonant mode isused before plasma formation and a drive resonant mode is used to powerthe plasma to steady state. The DWIPL has a cylindrical body wherein aredisposed a bulb, a drive probe, and a feedback probe. A combinedamplifier and control circuit connects the drive and feedback probes.

FIG. 15B schematically depicts an alternative configuration of the FIG.15A embodiment wherein the feedback probe is connected to the driveprobe by first and second diplexers and first and second combinedamplifiers and control circuits.

FIG. 16 is a block diagram of a first configuration of the FIGS. 11A,11B, 15A and 15B combined amplifier and control circuit.

FIG. 17 is a block diagram of a second configuration of the FIGS. 11A,11B, 15A and 15B combined amplifier and control circuit.

FIG. 18 is a block diagram of a configuration of the FIGS. 12, 13, 14Aand 14B combined amplifier and control circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention is open to various modifications andalternative constructions, the preferred embodiments shown in thedrawings will be described herein in detail. It is to be understood,however, there is no intention to limit the invention to the particularforms disclosed. On the contrary, it is intended that the inventioncover all modifications, equivalences and alternative constructionsfalling within the spirit and scope of the invention as expressed in theappended claims.

As used herein, the terms “dielectric waveguide integrated plasma lamp”,“DWIPL”, “microwave energized plasma lamp with solid dielectricwaveguide”, and “lamp” are synonymous, the term “lamp body” issynonymous with “waveguide body.” The term “probe” herein is synonymouswith “feed” in the '718 application. The term “power”, i.e., energy perunit time, is used herein rather than “energy” as in the '718application. The terms “lamp chamber” and “hole” herein are synonymouswith “cavity” in the '718 application, and are used in describingconstruction details, such as seals and materials, of the several DWIPLembodiments disclosed. A “lamp chamber” is defined herein as areceptacle, i.e., a hole, in a waveguide body having an aperture in abody surface which typically is coplanar with a waveguide surfaceexposed to the environment. The generic term “bulb” denotes (type-A) aself-enclosed, discrete structure containing a fill mixture andpositioned within a lamp chamber; or (type-B) a “bulb envelope,” viz., achamber containing a fill mixture sealed from the environment by awindow or lens. As used herein, the term “fill” is synonymous with “fillmixture.” The term “self-enclosed bulb” is specific to type-A. The term“cavity” is used herein when describing microwave technology-relateddetails such as probe design, coupling and resonant modes. From anelectromagnetic point of view a DWIPL body is a resonant cavity. Thischange in terminology from the '718 application was first made in the'340 application. For simplicity and to facilitate comparison, most ofthe lamp chambers and/or bulbs of the lamp embodiments disclosed hereinare cylindrical. However, other shapes such as rectangular prisms andellipsoids are feasible.

FIG. 1, adapted from FIG. 1 of the '028, '718, '340 and '788applications, shows a dielectric waveguide integrated plasma lamp with abulb envelope made of a transparent dielectric material rather than anopaque material such as a ceramic. DWIPL 101 includes a source 115 ofmicrowave radiation, a waveguide 103 having a body 104 consistingessentially of solid dielectric material, and a drive probe 117 couplingthe source 115 to the waveguide, which is in the shape of a rectangularprism determined by opposed sides 103A, 103B, and opposed sides 103C,103D generally transverse to sides 103A, 103B. DWIPL 101 furtherincludes a type-B bulb 107 disposed proximate to side 103A andpreferably generally opposed to probe 117, containing a fill 108including a “starting” gas 108G, such as a noble gas, and a lightemitter 108E, which when receiving microwave power at a predeterminedoperating frequency and intensity forms a plasma and emits light. Source115 provides microwave power to waveguide 103 via probe 117. Thewaveguide contains and guides the energy flow to a lamp chamber 105,depending from side 103A into body 104, in which bulb 107 is closelyreceived. This energy flow frees electrons from the starting gas atoms,thereby creating a plasma. In many cases the light emitter is solid atroom temperature. It may contain any one of a number of elements orcompounds known in the art, such as sulfur, selenium, a compoundcontaining sulfur or selenium, or a metal halide such as indium bromide.The starting plasma vaporizes the light emitter, and the microwavepowered free electrons excite the light emitter electrons to higherenergy levels. De-excitation of the light emitter electrons results inlight emission. Use of a starting gas in combination with a solid lightemitter is not a necessity; a gas fill alone, such as xenon, can be usedto start the plasma and to emit light. The preferred operating frequencyrange for source 115 is from about 0.25 to about 30 GHz. Source 115 maybe thermally isolated from bulb 107 which during operation typicallyreaches temperatures between about 700° C. and about 1000° C. thusavoiding degradation of the source due to heating. Preferably, thewaveguide body provides a substantial thermal mass which aids efficientdistribution and dissipation of heat and provides thermal isolationbetween the lamp and source. Additional thermal isolation of the sourcemay be accomplished by using an insulating material or vacuum gapoccupying an optional space 116 between source 115 and waveguide 103.When the space 116 is included, appropriate microwave probes are used tocouple the source to the waveguide.

Due to mechanical and other considerations such as heat, vibration,aging and shock, contact between the probe 117 and waveguide 103preferably is maintained using a positive contact mechanism 121, shownin FIG. 1 as a spring-loaded device. The mechanism provides a constantpressure by the probe on the waveguide to minimize the possibility thatmicrowave power will be reflected back through the probe rather thanentering the waveguide. In providing constant pressure, the mechanismcompensates for small dimensional changes in the probe and waveguidethat may occur due to thermal heating or mechanical shock. Preferably,contact is made by depositing a metallic material 123 directly on thewaveguide at its point of contact with probe 117 so as to eliminate gapsthat may disturb the coupling.

While the waveguide body 104, being of dielectric material, can byitself confine the microwave field to resonant modes within it,presenting the field with a conducting boundary condition at eachexternal body surface is desirable because the confined field amplitudeis increased, improving lamp efficiency, and the evanescent fieldoutside the waveguide body, characteristic of dielectric waveguides, isattenuated. Both the increased confined field amplitude and attenuatedevanescent field make oscillation inside the waveguide body lesssensitive to the outside environment, and suppress stray microwaveinterference. A conducting boundary condition can be effected in twoways, either singly or in combination. Sides 103A, 103B, 103C, 103D ofwaveguide 103, with the exception of those surfaces depending from side103A into body 104 which form lamp chamber 105, can be coated with athin metallic coating 119 which reflects microwaves in the operatingfrequency range. Alternatively, a tightly fitting metallic heatsink canserve the same purpose. A preferred coating is silver. Preferredmaterials for the heatsink include copper and aluminum.

Bulb 107 includes a wall 109 consisting of a transparent dielectricmaterial, preferably quartz, and is determined by a concavely arcuateinterior surface 110A, a generally cylindrical exterior surface 110B,and a generally planar bottom surface 110C. A window 111 attached toside 103A using a seal 113 hermetically seals and in combination withwall 109 determines a bulb envelope 127 which contains the fill 108,i.e., the emitter 108E and starting gas 108G. Because window 111 issealed to a waveguide surface rather than to the wall 109, matching thecoefficient of thermal expansion (CTE) of the materials used for thewindow and wall is not critical. Consequently, sapphire, which has highlight transmissivity, is a feasible material for window 111. Preferably,surface 110A is contoured to maximize the amount of light reflected outof bulb envelope 127 through window 111. Window 111 may include a lensto collect and focus the emitted-light. During operation when wall 109may reach temperatures of up to about 1000° C., body 104 acts as aheatsink because wall surfaces 110B, 110C are in thermal contact withthe waveguide body 104. Effective heat dissipation from body 104 isachieved by attaching a plurality of heat-sinking fins 125 to sides103A, 103C and 103D. Much of the energy absorbed by a plasma eventuallyappears as heat. Because wall 109 is transparent, such heat continuallyexits envelope 127 in the form of infrared and visible radiationabsorbed by body 104. Compared to a similar bulb envelope with a ceramicwall, a relatively large amount of power must be provided to maintainthe plasma temperature. Consequently, lamp 101 has low efficiency.

High resonant energy within the waveguide body 104, corresponding to ahigh Q-value in the body (where Q is the ratio of the operatingfrequency to the frequency width of the resonance), results in highevanescent leakage of microwave energy into chamber 105. Such leakageleads to quasi-static breakdown of the gas within envelope 127, therebygenerating the first free electrons. The oscillating energy of the freeelectrons scales as Iλ², where I is the circulating intensity of themicrowave energy and λ is the wavelength. Thus, the higher the microwaveenergy, the greater is the oscillating energy of the free electrons. Bymaking the oscillating energy greater than the ionization potential ofthe gas, electron-neutral collisions result in efficient build-up ofplasma density.

Once a plasma is formed and the incoming power is absorbed, thewaveguide body's Q-value drops due to the conductivity and absorptionproperties of the plasma. The drop in Q-value is generally due to achange in the impedance of the waveguide. After plasma formation, thepresence of the plasma in the chamber makes the chamber absorptive tothe resonant energy, thus changing the waveguide impedance. This changein impedance is effectively a reduction in the overall reflectivity ofthe waveguide. By matching the reflectivity of the probe to be close tothe reduced reflectivity of the waveguide, a relatively low netreflection back into the energy source is realized.

FIG. 2, which is FIG. 3B in the '028, '718 and '788 applications,illustrates a type-A (i.e., self-enclosed) bulb 140 disposed within alamp chamber 142 in a dielectric waveguide body 144. Bulb 140 includes agenerally cylindrical wall 146 of opaque dielectric material terminatingupwardly in a circumferential lip 148 having lower and upper surfaces148L, 148U, respectively, and attached to an opaque bottom 150. Lowersurface 148L is hermetically sealed by a seal 152 to a bulb supportstructure 154 attached to body 104. Bulb 140 further includes a window156 hermetically sealed by a seal 158 to upper surface 148U. Embedded insupport structure 154 is an access seal 160 through which air isevacuated from the chamber 142. Preferably, wall 146, lip 148 and bottom150 are made of alumina. Support structure 154 is made of materialhaving high thermal conductivity, such as alumina, to efficientlydissipate heat from the bulb. Once a vacuum is established in chamber142, heat transfer between the bulb 140 and waveguide body 144 issubstantially reduced.

FIG. 3A illustrates a type-A bulb and lamp chamber embodiment similar tothe type-B embodiment shown in FIG. 1. A self-enclosed bulb 170 isclosely received within a lamp chamber 172 in a dielectric waveguidebody 174. Bulb 170 includes a generally cylindrical wall 176 oftransparent dielectric material attached to a bottom 178 made of thesame material. Wall 176 has a circumferential upper edge 176Uhermetically sealed to a transparent window 180 by a seal 182. Window180 either is made of the same material as the wall and bottom or has aCTE which is very close to the CTE of that material. Preferably, thewall, bottom and window are made of quartz. The exterior surfaces 176E,178E, respectively, of wall 176 and bottom 178 are in thermal contactwith the surfaces of lamp chamber 172 contiguous to them, surfaces 172A,172B, respectively. Preferably, wall 176 has a thickness in a rangebetween one millimeter and ten millimeters. As in the FIG. 1 embodiment,heat from the plasma exits the bulb wall and bottom and is absorbed bythe waveguide body 174. Thus this “bulb cavity” embodiment has lowefficiency.

FIG. 3B illustrates a bulb and lamp chamber embodiment which differsfrom the FIG. 3A embodiment in one respect. The exterior surfaces 192E,194E, respectively, of generally cylindrical wall 192 and bottom 194 ofself-enclosed bulb 190 are coated with a thin-film, multi-layerdielectric coating 200 which allows the plasma to retain a significantfraction of its emission spectrum at its steady state operatingtemperature by reflecting radiation exiting the wall 192 and bottom 194back into the bulb. It should be emphasized that the coating is not madeof reflective material which, in general, prevents microwave power fromheating the light-emitting plasma. Tailored, broadband reflectivity overthe emission range of the plasma is instead achieved by interferenceamong electromagnetic waves propagating through thin-film layerspresenting refractive index changes at length-scales on the order oftheir wavelength. The number of layers and their individual thicknessesare the primary design variables. This is a well understood technologyin the optical industry [see Chapters 5 and 7, H. A. McLeod, “Thin-FilmOptical Filters,” 3rd edition, Institute of Physics Publishing (2001)],and such coatings having a reflectivity spectrum suitable for thepresent invention are available commercially. For ruggedness in theharsh environment proximate to bulb 190, a preferred embodiment ofcoating 200 consists of layers of silicon dioxide (SiO₂), which istransparent for wavelengths between 0.12 μm and 4.5 μm. Anotherpreferred embodiment consists of layers of titanium dioxide (TiO₂),which is transparent to wavelengths between 0.43 μm and 6.2 μm. FIG. 9shows the reflectance spectrum 202A, 202B, respectively, for lightincident at 30 degrees off normal and at normal incidence on a SiO₂multi-layer substrate. This coating was formulated according to ourspecifications by ZC&R Coatings for Optics Inc. of Torrance, Calif.Typically, coatings used in the present invention have approximately 10to 100 layers with each layer having a thickness in a range between 0.1μm and 10 μm. It is expected that a coating on the interior surface of abulb would not survive the plasma environment.

FIG. 4A illustrates a bulb and lamp chamber embodiment similar to theFIG. 2 embodiment. A self-enclosed bulb 210 is disposed within a lampchamber 212 in a dielectric waveguide body 214. Bulb 210 includes agenerally cylindrical wall 216 of transparent dielectric materialextending upwardly in a circumferential lip 218 having lower and uppersurfaces 218L, 218U, respectively, and attached to a transparent bottom220. Lower surface 218L is hermetically sealed by a seal 222 to a bulbsupport structure 224 attached to body 214. Bulb 210 further includes awindow 226 hermetically sealed by a seal 228 to upper surface 218U.Preferably, wall 216, lip 218, bottom 220 and window 226 are made ofquartz. Preferably, support structure 224 is made of dielectric ormetallic material having high thermal conductivity, such as,respectively, alumina or copper. The support structure 224 can betransparent to varying degrees in the emission band of the plasma atoperating conditions, or opaque. In contrast to the FIG. 2 embodiment,lamp chamber 212 is filled with air rather than evacuated. A lampconstructed according to this embodiment would have lower efficiencybecause heat lost from the bulb would not be recycled.

FIG. 4B illustrates a bulb and lamp chamber embodiment which differsfrom the FIG. 4A embodiment only in that the exterior surfaces 232E,234E, respectively, of generally cylindrical wall 232 and bottom 234 ofself-enclosed bulb 230 are coated with a thin-film, multi-layerdielectric coating 240 which allows the plasma to retain a significantfraction of its emission spectrum at its steady state operatingtemperature by reflecting radiation exiting the wall 232 and bottom 234back into the bulb.

FIG. 4C illustrates a bulb and lamp chamber embodiment wherein theexterior surfaces 252E, 254E, respectively, of generally cylindricalwall 252 and bottom 254 of self-enclosed bulb 250 are uncoated, andgenerally cylindrical wall surface 256 and generally planar bottomsurface 258 of lamp chamber 260 are coated with a thin-film, multi-layerdielectric coating 262. Radiation exiting the wall 252 and bottom 254 isreflected from coated surfaces 256 and 258 into bulb 250, therebyrecycling heat energy.

FIG. 4D illustrates a bulb and lamp chamber embodiment wherein theexterior surfaces 272E, 274E, respectively, of generally cylindricalwall 272 and bottom 274 of self-enclosed bulb 270 are coated with afirst thin-film, multi-layer dielectric coating 270C, and generallycylindrical wall surface 276 and generally planar bottom surface 278 oflamp chamber 280 are coated with a second thin-film, multi-layerdielectric coating 280C. Radiation exiting the wall 272 and bottom 274not reflected from coated surfaces 272E and 274E, i.e., passing throughcoating 270C, may be reflected from the coated chamber surfaces 276, 278back through coating 270C and into the bulb 270. It should be noted thatthe coatings on the bulb and chamber surfaces need not be of identicaldesign, but may have their spectral characteristics tailored to optimizeboth thermal efficiency and light output.

Referring to FIGS. 5A and 5B, a lamp chamber 290 has a surface 290Sbounding a surrounding wall 290W of dielectric material which may or maynot be continuous with the dielectric material 290D forming thewaveguide body, and an open aperture 290A. Surface 290S taperssymmetrically to a chamber bottom 290B, and is shaped to direct lightemitted by the plasma in the bulb so as to satisfy ray-divergencespecifications levied by the optical system receiving the lamp's outputradiation. Such specifications would not necessarily call for eitherstrictly convergent or strictly parallel rays; rays forming a limitednumerical aperture are the most likely requirement. In practice, aparaboloidal or an ellipsoidal shape can be used as a starting designpoint for a lamp chamber surface. The surface shape would then beoptimized using commercial ray-tracing software, taking into account thefinite emission volume of the plasma in the bulb, geometric constraintsimposed by a pedestal or other mount, and constraints imposed bymanufacturing processes. Suitable software products include ZEMAX™,available from Zemax Development Corporation of San Diego, Calif., andCODE-V™, available from Optical Research Associates of Pasadena, Calif.FIG. 5A shows two rays R1, R2 emanating from self-enclosed bulb 292 andreflecting off surface 290S. Reflection is due to the naturalreflectivity of the chamber wall material, which preferably is alumina.As shown in FIG. 5B, bulb 292 includes a generally cylindrical,transparent wall 294 attached to a bottom 296, and a window 298hermetically sealed to wall 294 by a seal 300. Preferably, the wall,bottom and window are made of quartz. Bulb 292 is mounted on a ceramicpedestal 302 by attaching generally planar bottom surface 296B of bottom296 to generally planar top surface 302T of pedestal 302 with a firstlayer 304A of high purity, high temperature, fast-cure ceramic adhesivesuch as RESBOND™ 940HT or 989, both alumina-oxide based compoundsavailable from Cotronics Corp. of Brooklyn, N.Y. Bottom surface 302B ofpedestal 302 is attached to wall 290W proximate to chamber bottom 290Bwith a second layer 304B of the ceramic adhesive.

FIG. 5C shows a modification of the FIG. 5A bulb and lamp chamberembodiment wherein the surface 310S of lamp chamber 310 is coated with athin-film, multi-layer dielectric coating 312 such as described above.Radiation exiting bulb 314 through generally cylindrical, transparentwall 316 is reflected from coated surface 310S so heat energy cannot berecycled in the bulb.

FIG. 5D shows a modification of the FIG. 5A bulb and lamp chamberembodiment wherein the exterior surfaces 322E, 324E, respectively, ofgenerally cylindrical, transparent wall 322 and window 324 of bulb 320are coated with a thin-film, multi-layer dielectric coating 326. Coating326 can be tailored so that most of the radiation from the plasma isreflected back into the bulb, providing high efficiency, while radiationin at least one selected spectral band escapes and is reflected fromuncoated surface 328S of lamp chamber 328.

FIG. 5E shows a modification of the FIG. 5A bulb and lamp chamberembodiment wherein the exterior surfaces 332E, 334E, respectively, ofgenerally cylindrical, transparent wall 332 and window 334 of bulb 330are coated with a thin-film, multi-layer dielectric first coating 336A,and surface 338S of lamp chamber 338 is coated with a thin-film,multi-layer dielectric second coating 336B. Coatings 336A and 336B canbe tailored so that most of the excited plasma's emission spectrum isreflected back into the bulb to recycle heat energy, while thoseportion(s) of the spectrum transmitted through coating 336A areselectively reflected from coated surface 336B so that only opticallyuseful light is directed outwardly. This embodiment provides highefficiency and also allows tailoring coatings 336A and 336B to providethe precise wavelength(s) or spectral band(s) desired.

FIG. 5F shows a partial-lens cover 338 which can be used in conjunctionwith any of the FIGS. 5A-E embodiments. Cover 338, which totally coversthe chamber aperture, includes an inner lens portion 338L and an outerplanar portion 338P, each made of a transparent material such as quartz.Planar portion 338P is attached by an adhesive layer 333 to thewaveguide wall surface 339 circumscribing the aperture. The opticalprescription of the lens and the area-fraction of the lens vis-a-visplanar portions are design parameters determined using commercialray-tracing software. Partial lens-cover 338 manipulates the divergenceof rays emanating from the bulb. A fraction of the bulb's direct rays,shown schematically as rays R1, R2, are focused by lens portion 338L,while a fraction of the bulb's reflected rays, shown schematically asrays R3, R4, pass undeflected through planar portion 338P. A fraction ofthe lamp's direct rays will escape unfocused from planar portion 338P,while a fraction of the reflected rays will be wrongly focused by lensportion 338L. However, considering that the position of the bulb in thechamber and the chamber bottom shape can potentially be subject toform-factor constraints such that they cannot form geometries ideal forconverging the bulb's output radiation, a partial-lens cover can in suchcircumstances improve a lamp's overall optical divergence properties.

Thin-film, multi-layer coatings applied to the exterior surfaces ofplasma bulbs made of quartz or a similar material would undergo manycycles of heating to temperatures that could approach 1000° C. andcooling to room temperature. The longevity of such coatings remains tobe determined. FIGS. 6A and 6B illustrate an embodiment which obviatesthis potential problem. A generally cylindrical, transparent bulb 340made of quartz or a similar material is surrounded by first and secondC-shaped half-cylinders 342A, 342B, respectively, having interiorsurfaces 344A, 344B, respectively, and pairs of first and second ends346A, 348A and 346B, 348B, respectively. Surfaces 344A and 344B arecoated with a thin-film, multi-layer coating 350. Half-cylinders 342A,342B, which may or may not be transparent, are made of a material havinga CTE similar to that of coating 350. Ends (346A, 346B) and (348A, 348B)are joined, using a high purity, high temperature, fast-cure ceramicadhesive such as RESBOND™ 940HT or 989 alumina-oxide based compound, toform a generally cylindrical sleeve 352 concentric about bulb 340.Preferably, sleeve 352 is separated from exterior surface 340E of bulb340 by an air gap 354. Alternatively, there is no air gap. Becausecoating 350 is on surfaces 344A and 344B rather than on bulb surface340E, the coating is not directly subjected to extreme temperaturevariation due to the bulb's heating-cooling cycle. Moreover, it issubjected to the mechanical stress of the matched-CTE sleeve rather thanthat of the bulb.

FIGS. 7A and 7B illustrate an embodiment similar to that in FIGS. 6A,6B, wherein C-shaped half-cylinders 360A, 360B, respectively, are madeof quartz or a similar transparent material and have exterior surfaces362A, 362B, respectively, coated with a thin-film, multi-layer coating364. A sleeve 366 formed by joining the half-cylinders 360A, 360Bpreferably is separated from exterior surface 368E of bulb 368 by an airgap 370. Alternatively, there is no air gap. Because coating 364 isinsulated from bulb 368 by sleeve 366 and air gap 370, coating 364 issubjected to much less temperature variation than coating 350.

The two-piece sleeves shown in FIGS. 6A, 6B and 7A, 7B can circumvent apotential problem in the manufacture of the thin-film, multi-layercoating. Tailoring the reflective properties of a multi-layer coatingdepends on being able to finely control the thickness of the individuallayers. While geometric constraints on the material beams used indeposition processes typically employed to apply such coatings may makeit impractical to simultaneously coat all surfaces of a cylindricalsleeve, simultaneous coating of both half-cylinders can be achieved.

FIGS. 8A and 8B illustrate an embodiment wherein a generallycylindrical, one-piece sleeve 380 made of quartz or a similartransparent material has an exterior surface 382A coated with athin-film, multi-layer coating 384. Interior surface 382B of sleeve 380preferably is separated from exterior surface 386E of bulb 386 by an airgap 388. Alternatively, there is no air gap. Coating 384 is insulatedfrom bulb 386 by sleeve 380 and air gap 388, so is subjected to muchless temperature variation than coating 350 and about as muchtemperature variation as coating 364.

We have found that for the bulb and lamp chamber embodiments shown inFIGS. 3A-B and 5A-F, whether or not the bulb is enclosed in any of theFIGS. 6A-B, 7A-B, 8A-B sleeves, intimate mechanical contact between thebulb and/or sleeve and the surrounding support structure is ofteninsufficient to provide adequate waste heat removal through conduction.In such cases, use of at least one high purity, high temperature,fast-cure ceramic adhesive layer between a bulb and the supportstructure (see FIG. 5B) or between sleeve outer surfaces and the supportstructure is necessary to provide the required heat conduction.Preferred adhesives are RESBOND 940HT or 989. For the bulb and lampchamber embodiments shown in FIGS. 4A-D, adequate waste heat removal canbe achieved by using a bulb support structure having high thermalconductivity.

A DWIPL can consist of a single integrated assembly including: awaveguide body with one or more lamp chambers each containing either abulb envelope sealed to the environment or a self-enclosed bulb; adriver circuit and driver circuit board; a thermal barrier separatingthe body and driver circuit; and an outer heatsink. Alternatively,separate packages are used for: (a) the lamp body and heatsink; and (b)the driver circuit and its heatsink. For a DWIPL utilizing two probes(see FIGS. 11A and 11B, and FIG. 6 of the '718 application), the bodyand driver circuit are connected by two RF power cables, one connectingthe output of the driver circuit to the body, and the other providingfeedback from the body to the driver circuit. The use of two separatepackages allows greater flexibility in the distribution of lamp heat andlamp driver heat.

When microwave power is applied from the driver circuit to the lampbody, it heats the fill mixture, melting and then vaporizing the salt orhalide, causing a large increase in the lamp chamber pressure. Dependingon the salt or halide used, this pressure can become as high as 400atmospheres, and the bulb temperature as high as 1000° C. Consequently,a seal attaching a window or lens to a lamp body side or the wall of aself-enclosed bulb must be extremely robust.

Electromagnetically, a DWIPL is a resonant cavity having at least onedrive probe supplying microwave power for energizing a plasma containedin at least one bulb. In the following portion of the detaileddescription “cavity” denotes a DWIPL body. As disclosed in the '718application, a “bulb” may be a separate enclosure containing a fillmixture disposed within a lamp chamber, or the chamber itself may be thebulb. To provide optimal efficiency, a bulb preferably is located at anelectric field maximum of the resonant cavity mode being used. Howeverthe bulb can be moved away from a field maximum at the cost ofadditional power dissipated by the wall and cavity. The location of thedrive probe is not critical, as long as it is not at a field minimum,because the desired coupling efficiency can be achieved by varying probedesign parameters, particularly length and shape. FIGS. 10A and 10Bschematically show two cylindrical lamp configurations 400A, 400B,respectively, both operating at the fundamental cylindrical cavity mode,commonly known as TM_(0,1,0), and having a bulb 402A, 402B,respectively, located at the single electric field maximum. Dashedcurves 401A, 401B show, respectively, the electric field distribution inthe cavity. In FIG. 10A, a drive probe 404A is located at the fieldmaximum. In FIG. 10B, drive probe 404B is not located at the fieldmaximum; however, it contains a longer probe which provides the samecoupling efficiency as probe 404A. Although the TM_(0,1,0) mode is usedhere as an example, higher order cavity modes, including but not limitedto transverse electric field (“TE”) and transverse magnetic field (“TM”)modes, can also be used.

Drive probe design is critical for proper lamp operation. The probe mustprovide the correct amount of coupling between the microwave source andlamp chamber to maximize light emitting efficiency and protect thesource. There are four major cavity loss mechanisms reducing efficiency:chamber wall dissipation, dielectric body dissipation, plasmadissipation, and probe coupling loss. As defined herein, probe couplingloss is the power coupled out by the drive probe and other probes in thecavity. Probe coupling loss is a major design consideration because anyprobe can couple power both into and out of the cavity. If the couplingbetween the source and cavity is too small, commonly known as“under-coupling”, much of the power coming from the source will notenter the cavity but be reflected back to the source. This will reducelight emission efficiency and microwave source lifetime. If initiallythe coupling between the source and cavity is too large, commonly knownas “over-coupling”, most of the power from the source will enter thecavity. However, the cavity loss mechanisms will not be able to consumeall of the power and the excess will be coupled out by the drive probeand other probes in the cavity. Again, light emission efficiency andmicrowave source lifetime will be reduced. In order to maximize lightemission efficiency and protect the source, the drive probe must providean appropriate amount of coupling such that reflection from the cavityback to the source is minimized at the resonant frequency. Thiscondition, commonly known as “critical coupling”, can be achieved byadjusting the configuration and location of the drive probe. Probedesign parameters depend on the losses in the cavity, which depend onthe state of the plasma and the temperature of the lamp body. As theplasma state and/or body temperature change, the coupling and resonantfrequency will also change. Moreover, inevitable inaccuracies duringDWIPL manufacture will cause increased uncertainty in the coupling andresonant frequency.

It is not practical to adjust probe physical parameters while a lamp isoperating. In order to maintain as close to critical coupling aspossible under all conditions, a feedback configuration is required (seeFIG. 6 of the '718 application), such as lamp configurations 410A, 410Bshown, respectively, in FIGS. 11A and 11B for a rectangular prism-shapedcavity and a cylindrical cavity. A second “feedback” probe 412A, 412B,respectively, is introduced into a cavity 414A, 414B, respectively.Feedback probe 412A, 412B, respectively, is connected to input port416A, 416B, respectively, of a combined amplifier and control circuit(ACC) 418A, 418B, respectively, and a drive probe 420A, 420B,respectively, is connected to ACC output port 422A, 422B, respectively.Each configuration forms an oscillator. Resonance in the cavity enhancesthe electric field strength needed to create the plasma and increasesthe coupling efficiency between the drive probe and bulb. Both the driveprobe and feedback probe may be located anywhere in the cavity exceptnear an electric field minimum for electric field coupling, or amagnetic field minimum for magnetic field coupling. Generally, thefeedback probe has a lesser amount of coupling than the drive probebecause it samples the electric field in the cavity with minimumincrease in coupling loss.

From a circuit perspective, a cavity behaves as a lossy narrow bandpassfilter. The cavity selects its resonant frequency to pass from thefeedback probe to the drive probe. The ACC amplifies this preferredfrequency and puts it back into the cavity. If the amplifier gain isgreater than the insertion loss at the drive probe entry port vis-a-visinsertion loss at the feedback probe entry port, commonly known as S₂₁,oscillation will start at the resonant frequency. This is doneautomatically and continuously even when conditions, such as plasmastate and temperature, change continuously or discontinuously. Feedbackenables manufacturing tolerances to be relaxed because the cavitycontinually “informs” the amplifier of the preferred frequency, soaccurate prediction of eventual operating frequency is not needed foramplifier design or DWIPL manufacture. All the amplifier needs toprovide is sufficient gain in the general frequency band in which thelamp is operating. This design ensures that the amplifier will delivermaximum power to the bulb under all conditions.

In order to maximize light emission efficiency, a drive probe isoptimized for a plasma that has reached its steady state operatingpoint. This means that prior to plasma formation, when losses in acavity are low, the cavity is over-coupled. Therefore, a portion of thepower coming from the microwave source does not enter the cavity and isreflected back to the source. The amount of reflected power depends onthe loss difference before and after plasma formation. If thisdifference is small, the power reflected before plasma formation will besmall and the cavity will be near critical coupling. A feedbackconfiguration such as shown in FIG. 11A or 11B will be sufficient tobreak down the gas in the bulb and start the plasma formation process.However, in most cases the loss difference before and after plasmaformation is significant and the drive probe becomes greatlyover-coupled prior to plasma formation. Because much of the power isreflected back to the amplifier, the electric field strength may not belarge enough to cause gas breakdown. Also, the large amount of reflectedpower may damage the amplifier or reduce its lifetime.

FIG. 12 shows a lamp configuration 430 which solves the drive probeover-coupling problem wherein a third “start” probe 432, optimized forcritical coupling before plasma formation, is inserted into a cavity434. Start probe 432, drive probe 436, and feedback probe 438 can belocated anywhere in the cavity except near a field minimum. Power fromoutput port 440B of an ACC 440 is split into two portions by a splitter442: one portion is delivered to drive probe 436; the other portion isdelivered to start probe 432 through a phase shifter 444. Probe 438 isconnected to input port 440A of ACC 440. Both the start and drive probesare designed to couple power into the same cavity mode, e.g., TM_(0,1,0)for a cylindrical cavity as shown in FIG. 12. The splitting ratio andamount of phase shift between probes 436 and 432 are selected tominimize reflection back to the amplifier. Values for these parametersare determined by network analyzer S-parameter measurements and/orsimulation software such as High Frequency Structure Simulator (HFSS)available from Ansoft Corporation of Pittsburgh, Pa. In summary, thestart probe is critically coupled before plasma formation and the driveprobe is critically coupled when the plasma reaches steady state. Thesplitter and phase shifter are designed to minimize reflection back tothe combined amplifier and control circuit.

FIG. 13 shows a second lamp configuration 450 which solves the driveprobe over-coupling problem. Both start probe 452 and drive probe 454are designed to couple power into the same cavity mode, e.g., TM_(0,1,0)for a cylindrical cavity such as cavity 456. Configuration 450 furtherincludes a feedback probe 458 connected to input port 460A of an ACC460. The three probes can be located anywhere in the cavity except neara field minimum. Power from output port 460B of ACC 460 is delivered toa first port 462A of a circulator 462 which directs power from port 462Ato a second port 462B which feeds drive probe 454. Prior to plasmaformation, there is a significant amount of reflection coming out of thedrive probe because it is over-coupled before the plasma reaches steadystate. Such reflection is redirected by circulator 462 to a third port462C which feeds the start probe 452. Before plasma formation, the startprobe is critically coupled so that most of the power is delivered intothe cavity 456 and start probe reflection is minimized. Only aninsignificant amount of power goes into port 462C and travels back toACC output port 460B. Power in the cavity increases until the fillmixture breaks down and begins forming a plasma. Once the plasma reachessteady state, the drive probe 454 is critically coupled so reflectionfrom the drive probe is minimized. At that time, only an insignificantamount of power reaches the now under-coupled start probe 452. Althoughthe start probe now has a high reflection coefficient, the total amountof reflected power is negligible because the incident power isinsignificant. In summary, the start probe is critically coupled beforeplasma formation and the drive probe is critically coupled when theplasma reaches steady state. The circulator directs power from port 462Ato 462B, from port 462B to port 462C, and from port 462C to port 462A.

FIGS. 14A and 14B show third and fourth lamp configurations 470A, 470Bwhich solve the drive probe over-coupling problem. A “start” cavity modeis used before plasma formation, and a separate “drive” cavity mode isused to power the plasma to its steady state and maintain that state.Start probe 472A, 472B, respectively, operates in the start cavity mode,and drive probe 474A, 474B, respectively, operates in the drive cavitymode. As indicated by dashed curves 471A and 471B, preferably the drivecavity mode is the fundamental cavity mode and the start cavity mode isa higher order cavity mode. This is because normally it requires morepower to maintain the steady state plasma with the desired light outputthan to break down the gas for plasma formation. Therefore it is moreeconomical to design a DWIPL so the high power microwave source operatesat a lower frequency. For a cylindrical cavity such as cavities 476A and476B, the start probe 472A, 472B, respectively, can be criticallycoupled at the resonant frequency of the TM_(0,2,0) mode before plasmaformation, and the drive probe 474A, 474B, respectively, can be coupledat the resonant frequency of the TM_(0,1,0) mode after the plasmareaches steady state. The feedback probe can be located anywhere in thecavity except near a field minimum of the drive cavity mode or a fieldminimum of the start cavity mode. The start probe can be locatedanywhere in the cavity except near any field minima of the start cavitymode. The drive probe should be located near or at a field minimum ofthe start cavity mode but not near a field minimum of the drive cavitymode. This minimizes the coupling loss of the drive probe before plasmaformation so that the electric field in the cavity can reach a highervalue to break down the gas. A diplexer 478A, 478B, respectively, isused to separate the two resonant frequencies. In FIG. 14A, a single ACC480 connected at its output 480B to diplexer 478A is used to power bothcavity modes. The two frequencies are separated by diplexer 478A and fedto the start probe 472A and drive probe 474A. Feedback probe 482A isconnected to input port 480A of ACC 480. In FIG. 14B, two separateamplifiers 490, 492 with output ports 490B, 492B, respectively, are usedto power the two cavity modes independently. Diplexer 478B separates thetwo frequencies coming out of feedback probe 482B. In summary, the startprobe operates in one cavity mode and the drive probe operates in adifferent mode. The feedback probe can be located anywhere in the cavityexcept near a field minimum of either mode. The start probe can belocated anywhere in the cavity except near a field minimum of the startcavity mode. The drive probe should be located near or at a fieldminimum of the start cavity mode but not near a field minimum of thedrive cavity mode.

An alternative to the approach shown in FIG. 14B is to split thefeedback probe 482B into two feedback probes, thereby eliminating theneed for diplexer 478B. The first feedback probe is located at a fieldminimum of the start cavity mode to couple out only the drive cavitymode, which is then amplified by ACC 490 connected at its output 490B todrive probe 474B. The second feedback probe is located at a fieldminimum of the drive cavity mode to couple out only the start cavitymode, which is then amplified by ACC 492 connected at its output 492B tostart probe 472B. Two separate feedback loops are implemented, with thefunction of the diplexer separating the drive and start cavity modesbeing replaced by proper placement of the two feedback probes.

FIGS. 15A and 15B show lamp configurations 500A, 500B, respectively,which do not include a start probe but utilize two separate cavitymodes. As indicated by curves 501A and 501B, respectively, in cavities502A and 502B, a relatively high order start cavity mode is used beforeplasma formation and a relatively low order drive cavity mode is used topower the plasma to steady state and maintain the state. Preferably, foreconomy and efficiency, the drive cavity mode again is the fundamentalcavity mode and the start cavity mode is a higher order cavity mode. Forexample, the TM_(0,2,0) mode of a cylindrical lamp cavity can be usedbefore plasma formation, and the TM_(0,1,0) mode can be used to maintainthe plasma in steady state. By utilizing two cavity modes, it ispossible to design a single drive probe that is critically coupled bothbefore plasma formation and after the plasma reaches steady state,thereby eliminating the need for a start probe. The feedback probe 504A,504B, respectively, can be located anywhere in the cavity except near afield minimum of either cavity mode. The drive probe 506A, 506B,respectively, should be located near a field minimum of the start cavitymode but not near a field minimum of the drive cavity mode. By placingthe drive probe near but not at a field minimum of the start cavitymode, the drive probe can be designed to provide the small amount ofcoupling needed before plasma formation and the large amount of couplingrequired after the plasma reaches steady state when the plasma lossgreatly increases. In FIG. 15A, a single ACC 510 having input and outputports 510A, 510B, respectively, is used to power both cavity modes. InFIG. 15B, two separate ACC's 512, 514 are used to power the two cavitymodes independently. A first diplexer 516B separates the two frequenciescoming out of feedback probe 504B and a second diplexer 518B combinesthe two frequencies going into drive probe 506B. In summary, the driveprobe is critically coupled at the start cavity mode resonant frequencybefore plasma formation and critically coupled at the drive cavity moderesonant frequency when the plasma reaches steady state. The feedbackprobe can be located anywhere in the cavity except near a field minimumof either cavity mode. The drive probe should be located near a fieldminimum of the start cavity mode but not near a field minimum of thedrive cavity mode.

The '718 application disclosed a technique for drive probe constructionwherein a metallic microwave probe is in intimate contact with the highdielectric material of the lamp body. This method has a drawback in thatthe amount of coupling is very sensitive to the exact dimensions of theprobe. A further drawback is that due to the large temperature variationbefore plasma formation and after the plasma reaches steady state, amechanism such as a spring is needed to maintain contact between theprobe and body. These constraints complicate the manufacturing processand consequently increase production cost. A technique which avoids bothproblems is to surround a metallic microwave probe extended into a lampbody with a dielectric material having a high breakdown voltage. (Due tothe large amount of power delivered within a limited space, the electricfield strength near the probe's tip will be very high; therefore a highbreakdown voltage material is required.) Typically, this material has alower dielectric constant than that of the dielectric material formingthe lamp body. The material acts as a “buffer” which desensitizes thedependency of coupling on probe dimensions, thereby simplifyingfabrication and reducing cost. Preferred buffer materials are TEFLON™and mullite, a refractory ceramic. The amount of coupling between themicrowave source and body can be adjusted by varying the location anddimensions of the probe, and the dielectric constant of the material. Ingeneral, if the probe length is less than a quarter of the operatingwavelength, a longer probe will provide greater coupling than a shorterprobe. Also, a probe placed at a location with a higher field willprovide greater coupling than a probe placed at a location where thefield is relatively low. This technique also is applicable to a startprobe or a feedback probe. The probe location, shape and dimensions canbe determined using network analyzer S-parameter measurements and/orsimulation software such as HFSS.

FIG. 16 shows a circuit 520 including an amplifier 522 and a controlcircuit 524, suitable for DWIPLs having only a drive probe 526 andfeedback probe 528 such as shown in FIGS. 11A, 11B, 15A and 15B, andexemplified here by lamp 530. The function of amplifier 522 is toconvert dc power into microwave power of an appropriate frequency andpower level so that sufficient power can be coupled into lamp body 532and lamp chamber 534 to energize a fill mixture and form alight-emitting plasma.

Preferably, amplifier 522 includes a preamplifier stage 536 with 20 to30 dB of gain, a medium power amplifier stage 538 with 10 to 20 dB ofgain, and a high power amplifier stage 540 with 10 to 18 dB of gain.Preferably, stage 536 uses the Motorola MIL21336, 3G Band RF LinearLDMOS Amplifier, stage 538 uses the Motorola MRF21030 Lateral N-ChannelRF Power MOSFET; and stage 540 uses the Motorola MRF21125 LateralN-Channel RF Power MOSFET. These devices as well as complete informationfor support and bias circuits are available from Motorola SemiconductorProducts Sector in Austin, Tex. Alternatively, stages 536, 538 and 540are contained in a single integrated circuit. Alternatively, stages 536and 538, and control circuit 524 are packaged together, and high powerstage 540 is packaged separately.

Amplifier 522 further includes a PIN diode attenuator 542 in series withstages 536, 538 and 540, preferably connected to preamplifier stage 536to limit the amount of power which the attenuator must handle.Attenuator 542 provides power control for regulating the amount of powersupplied to lamp body 532 appropriate for starting the lamp, operatingthe lamp, and controlling lamp brightness. Since the amplifier chainformed by stages 536, 538 and 540 has a fixed gain, varying theattenuation during lamp operation varies the power delivered to body532. Preferably, the attenuator 542 acts in combination with controlcircuit 524, which may be analog or digital, and an optical powerdetector 544 which monitors the intensity of the light emitted andcontrols attenuator 542 to maintain a desired illumination level duringlamp operation, even if power conditions and/or lamp emissioncharacteristics change over time. Alternatively, an RF power detector546 connected to drive probe 526, amplifier stage 540 and controlcircuit 524 is used to control the attenuator 542. Additionally, circuit524 can be used to control brightness, i.e., controlling the lampillumination level to meet end-application requirements. Circuit 524includes protection circuits and connects to appropriate sensingcircuits to provide the functions of over-temperature shutdown,over-current shutdown, and over-voltage shutdown. Circuit 524 can alsoprovide a low power mode in which the plasma is maintained at a very lowpower level, insufficient for light emission but sufficient to keep thefill mixture gas ionized. Circuit 524 also can shut down the lamp slowlyby increasing the attenuation. This feature limits the thermal shock alamp repeatedly experiences and allows the fill mixture to condense inthe coolest portion of the lamp chamber, promoting easier lamp starting.

Alternatively, attenuator 542 is combined with an analog or digitalcontrol circuit to control the output power at a high level during theearly part of the lamp operating cycle, in order to vaporize the fillmixture more quickly than can be achieved at normal operating power.Alternatively, attenuator 542 is combined with an analog or digitalcontrol circuit which monitors transmitted and/or reflected microwavepower levels through an RF power detector and controls the attenuator tomaintain the desired power level during normal lamp operation, even ifthe incoming power supply voltage changes due to variations in the acsupply or other loads.

FIG. 17 shows an alternative circuit 560 including an amplifier 562 anda control circuit 564, suitable for supplying and controlling power tothe body 566 and lamp chamber 568 of a DWIPL 570 having a drive probe572 and feedback probe 574, such as shown in FIGS. 11A, 11B, 15A and15B. A “starting” bandpass filter 580A and an “operating” bandpassfilter 580B, in parallel and independently selectable and switchable,are in series with the FIG. 16 amplifier chain and preferably, as inFIG. 16, on the input side of the chain. Filters 580A and 580B filterout frequencies corresponding to undesired resonance modes of body 566.By selecting and switching into the circuit a suitable filter bandpassusing first and second PIN diode switches 582A, 582B, the DWIPL 570 canoperate only in the cavity mode corresponding to the selected frequencyband, so that all of the amplifier power is directed into this mode. Apin diode attenuator 584 is connected between pin diode switch 582A andfeedback probe 574. By switching in filter 580A, a preselected firstcavity mode is enabled for starting the lamp. Once the fill mixture gashas ionized and the plasma begun forming, a selected second cavity modeis enabled by switching in filter 580B. For a short time, both filtersprovide power to the lamp to ensure that the fill mixture remains aplasma. During the period when both filters are switched in, both cavitymodes propagate through body 566 and the amplifier chain. When apredetermined condition has been met, such as a fixed time delay or aminimum power level, filter 580A is switched out, so that only thecavity mode for lamp operation can propagate through the amplifierchain. Control circuit 564 selects, deselects, switches in, and switchesout filters 580A and 580B, following a predetermined operating sequence.An optical power detector 586 connected to control circuit 564 performsthe same function as detector 544 in the FIG. 16 embodiment.

FIG. 18 shows a circuit 600 including an amplifier 602 and an analog ordigital control circuit 604, suitable for supplying and controllingpower to the body 606 and lamp chamber 608 of a DWIPL 610 having a driveprobe 612, a feedback probe 616 and a start probe 614, such as shown inFIGS. 12, 13, 14A and 14B. The feedback probe 616 is connected to input536A of preamplifier 536 through a PIN diode attenuator 618 and a filter620. The start probe 614 is designed to be critically coupled when lamp610 is off. To start the lamp, a small amount of microwave power isdirected into start probe 614 from preamplifier stage 536 or mediumpower stage 538 of the amplifier chain. The power is routed through abipolar PIN diode switch 622 controlled by control circuit 604. Switch622 is controlled to send RF microwave power to start probe 614 untilthe fill mixture gas becomes ionized. A sensor 624A monitors power usagewithin body 576, and/or a sensor 624B monitors light intensityindicative of gas ionization. A separate timer control circuit, which ispart of control circuit 604, allocates an adequate time for gasbreakdown. Once the gas has been ionized, control circuit 604 turns onswitch 622 which routes microwave power to high power stage 540 whichprovides microwave power to drive probe 612. For a short time, startprobe 614 and drive probe 612 both provide power to the lamp to ensurethat the fill mixture remains a plasma. When a predetermined conditionhas been met, such as a fixed time period or an expected power level,control circuit 604 turns off switch 622 thereby removing power to startprobe 614 so that the plasma is powered only by drive probe 612. Thisprovides maximum efficiency.

To enhance the Q-value (i.e., the ratio of the operating frequency tothe resonant frequency bandwidth) of the DWIPL 610 during starting, thecontrol circuit 604 can bias the transistors of high power stage 540 toan impedance that minimizes leakage out of probe 612 into stage 540. Toaccomplish this, circuit 604 applies a dc voltage to the gates of thetransistors to control them to the appropriate starting impedance.

While several embodiments for carrying out the invention have been shownand described, it will be apparent to those skilled in the art thatadditional modifications are possible without departing from theinventive concepts detailed herein. It is to be understood, therefore,there is no intention to limit the invention to the particularembodiments disclosed. On the contrary, it is intended that theinvention cover all modifications, equivalences and alternativeconstructions falling within the spirit and scope of the invention asexpressed in the appended claims.

1. A lamp comprising: (a) a waveguide having a body of a preselectedshape and dimensions, the body comprising at least one dielectricmaterial and having at least one surface determined by a waveguide outersurface, each said material having a dielectric constant greater thanapproximately 2; (b) a first microwave probe positioned within and inintimate contact with the body, adapted to couple microwave energy intothe body from a microwave source having an output and an input andoperating within a frequency range from about 0.25 GHz to about 30 GHzat a preselected frequency and intensity, the probe connected to thesource output, said frequency and intensity and said body shape anddimensions selected so that the body resonates in at least one resonantmode having at least one electric field maximum; (c) the body having alamp chamber depending from said waveguide outer surface and determinedby a chamber aperture and a chamber enclosure determined by a bottomsurface and at least one surrounding wall surface; (d) a transparent,dielectric bulb within the lamp chamber; and (e) a fill mixturecontained within the bulb which when receiving microwave energy from theresonating body forms a light-emitting plasma.